Mold apparatus for casting with downward unidirectional solidification

ABSTRACT

Unidirectionally solidified castings are produced by inverted solidification using a chill plate at the top of the mold and controlling the temperature gradient to cause solidification to occur from the top downwardly through the mold.

[45] Aug. 14, 1973 United States Patent [191 Copley et al.

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FOREIGN PATENTS OR APPLICATIONS [75] Inventors: Stephen M. Copley,Madison; 241,758 5/1881 Anthony F. Giamei, New Haven, 3,233,292 2/1966Kramer et both of Conn.

988,645 5/1951 France................................ 249/109 [73]Assignee: United Aircraft Corporation, East Hartford, Conn.

July 22, 1971 App]. No.: 165,302

Primary Examiner-Robert D. Baldwin Attorney-Charles A. Warren [22]Filed:

[57] ABSTRACT Unidirectionally solidified castings are produced bRelated US. Application Data y mverted solidification using a chillplate at the top of the mold and controlling the temperature gradient tocause solidification to occur from the top downwardly through the mold.

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[ [51] Int. B22d 27/04 164/60, 120, 127, 164/352, 353, 361, 338; 249/78,109

5 Claims, 4 Drawing Figures [58] Field of W mesaoeooaceeeeeeocaeoog v iw Patented Aug. 14, 1973 2 Sheets-Sheet l 4 La W Patented Aug. 14, 19732 Sheets-Sheet 2 MOLD APPARATUS FOR CASTING WITH DOWNWARD UNIDIRECTIONALSOLIDIFICATION This is a division of Ser. No. 872,562 filed Oct. 30,1969, now US. Pat. No. 3,598,172 for Process of Casting with DownwardUnidirectional Soldification.

BACKGROUND OF THE INVENTION Unidirectionally solidified castings ofthe'columnar grained type as described in VerSnyder US. Pat. No.3,260,505 and also of the type described in Piearcey Ser. No. 540,114,filed Feb. 17, 1966, US. Pat. No. 3,494,709 and assigned to a commonassignee, are generally formed by casting in a mold resting on a chillplate with a control of the thermal gradient to cause a controlledupward movement of the liquid-solid interface from the chill plateupward to the top of the mold. Such castings are generally satisfactoryalthough at times there is a density inversion during solidificationthat creates imperfections within or on the surface of the castings.

SUMMARY OF INVENTION One feature of this invention is the invertedunidirectional solidification of alloys in the mold for the purpose ofeliminating the imperfections resulting from the density inversion.Another feature is a mold assembly that makes possible the directionalsolidification of alloys from top to bottom with a controlled thermalgradient thereby to produce the desired crystalline structure and/orgrain growth.

BRIFF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical sectional viewthrough a mold construction.

FIG. 2 is a horizontal sectional view along the line 2-2 of FIG. 1.

FIG. 3 is a fragmentary view of a modification.

FIG. 4 is a wiring diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENT The shell mold apparatusincludes a central vertical feeder tube connected at the bottom bylateral arms 12 having passages 14 thereon. At the outer ends of thearms are vertical mold elements 16 extending parallel and in spacedrelation to the feeder tube and having an article forming cavity 18therein. This cavity is shown as being the shape of a turbine blade withan airfoil portion 20, a shroud portion 22 at the bottom end and a rootportion 24 at the top thereof. The shroud portion communicates through avertical passage 30 with the horizontal passage 14. The root portion ofthe article cavity 18 communicates through a helical crystal selectorpassage 32 with a growth zone cavity 34. A chill plate 36 is secured tothe mold at the top end of the cavity 34, the mold having a flange 38 atthis point by which the chill plate is attached.

As shown in FIG. 2, the mold is generally made up of a plurality ofvertical mold elements 16 so that a plurality of articles may be cast atone time. The mold may be made with a plurality of supporting feet 40.The mold is made by the usual shell mold technique using a wax or otherdisposable pattern over which successive layers of mold material areplaced and dried, with the finished mold cured by heating at which timethe pattern is melted out.

When the mold is ready for use, it is placed on a support plate 42resting on a bed of a heat insulating powered material 44 such asaluminum oxide. The mold is surrounded by a susceptor 46 which in turnis surrounded by vertically spaced induction heating coils 48 and 49which are selectively energized for controlling the temperature of themold. The susceptor and heaters extend at least from a point above thechill plate to a point substantially below the shroud portion to assurean accurate control of the thermal gradient from the chill to the bottomend of the vertical passage 30.

The central tube is surrounded by a heating coil 50, preferably aresistance coil, and this coil is shielded from the surrounding articleforming portions of the mold by a cylindrical shield 52. A graphitesleeve 53 may be positioned within the coil, as shown. The latter andthe shield 52 may rest on the lateral arms 12 and the coil may beincorporated in this shield, as shown. A sprue 54 positioned at the topof the tube directs molten alloy into the vertical pasage defined by thetube. The entire structure is preferably mounted in a vacuum chamber sothat the casting procedure may be done under vacuum or in an inertatmosphere.

The assemblage is used for example in producing parts such as turbineblades or vanes which operate in a high temperature environment underhigh stress and are thus cast from the so-called super alloys such asdecribed in the VerSnyder US. Pat. No. 3,260,505 or the Piearcey US.Pat. No. 3,494,709. The article forming portion of the mold and theadjacent portions thereof are heated to a temperature above the meltingtemperature of the alloy and the central tube is also raised to thissame temperature. A flow of water is maintained through the chill plateso that it is not melted during this heating process. When temperaturesare stablized the molten alloy, somewhat superheated, for example, aboutC, is poured into the sprue to fill the mold completely. A suitable vent56 in the chill plate permits the escape of entrapped gases within themold to assure complete filling.

The coils 48 and 49 may be connected, as shown in FIG. 4 to a source ofalternating current represented by the power leads 58. Energy to coil 49is controlled by a switch 60, and energy to coil 48 is controlled by arheostat 62 in order to provide a programmed rate of reduction of poweras determined by the solidification rate. The coil 50 may also beconnected to the power leads as by a switch 64. As stated later in thespecification, this coil is cut off from the supply when the downwardsolidification in the article forming portion of the mold is completed.

At the time the mold is filled, solidification of the alloy begins atthe water cooled copper chill plate. Up to this time both inductionheating coils 48 and 49 have been energized but as the mold is filledthe upper coil 49 is cut off as by the switch 60, FIG. 4 to begin thecooling of the mold from top to bottom. The escape of heat to the' chillplate causes the vertically downward growth of columnar grains of alloyin the growth cavity 34 and into the helical passage 32 where a singlecrystal is selected to continue its growth downwardly into the articleforming cavity thereby forming a single crystal article as described andclaimed in Piearcey US. Pat. No. 3,494,709.

The maintenance of heat from the resistance coil around the central tubekeeps the alloy in thistube molten, and the heat from induction coil 48keeps the alloy molten in the horizontal passage so that the metal inthe central tube maintains a positive hydrostatic pressure on theliquid-solid interface where the alloy is solidifying downwardly in thearticle cavity. This hydrostatic pressure increases as the liquid-solidinterface proceeds downwardly and thus the pressure on the mushy zoneincreases toward the bottom end of the article cavity.

As the liquid-solid interface moves downwardly during the castingoperation, the power in the lower heater 48 is reduced at a programmedrate as by the rheostat 62, FIG. 4 determined by the rate ofsolidification, and is then cut off to permit completion of thesolidification. Once the downward solidification in the article formingportion is complete, heat to the central tube heater is cut off so thatthe remainder of the alloy may solidify.

It has been found that in the usual formation of directionallysolidified articles from bottom to top there are liquid jets of thealloy that flow upwardly within the mushy zone caused by instabilityresulting from a density inversion in this zone. These jetsdetrimentally affect the proper solidification of the alloy within themushy zone. This may result in local areas of segregation trails rich inrejected solute which on the surface of the cast article are referred toas freckles. These trails often contain small randomly oriented grainsand are a defect which may make the cast article unacceptable. Suchfreckles or trails would be eliminated by the inverted solidificationabove described, since the mushy zone density profile will be stablewith respect to the gravitational field.

Referring now to P10. 3, the mold may be adapted for making columnargrained articles as in the VerSnyder patent. To accomplish this, thegrowth cavity 34' communicates directly with the root portion 24 so thatthe columnar growth that is started at the chill plate 36' and becomesparallel, vertically oriented columnar grains in the growth zone arepropagated downwardly through the article forming portion. Casting ofsuch articles by this inverted solidification process would be used toproduce acceptable columnar grained article such as turbine blades foruse in the highest temperature turbine stages of the engine.

We claim:

1. Apparatus for forming directionally solidified cast articles,including a mold having a filling cavity and an article forming cavityin horizontally spaced vertically positioned relation, and aninterconnecting passage between said cavities at the bottom, a chillplate closing the open upper end of the article forming cavity, heatingmeans for said filling cavity, and other heating means for said articleforming cavity, and means for selectively reducing the heating effect inportions of said other heating means between top and bottom thereof toproduce a controlled thermal gradient lower at the top than at thebottom of said article cavity.

2. Apparatus as in claim 1, including a container to receive the moldwith a granular material within the container and supporting the mold.

3. Apparatus as in claim 1 in which the article forming cavity of themold has a single crystal selector adjacent the upper end directly belowthe chill plate.

4. A shell mold apparatus for casting of directionally solidifiedarticles including an article mold portion having means adjacent the topfor starting the crystallization of the material therein as a singlecrystal, said mold portion having an open top end above said means,

a chill plate extending across and closing said open a parallel fillingmold portion open at the top for filling the mold, an interconnectingpassage mold portion connecting said articlev and filling portions atthe bottoms thereof, the top of said filling portion being higher thanthe open top end of the article portion,

induction heating means around the article mold portion for maintainingthis portion at a temperature higher than the melting temperature of thematerial to be cast in the mold, and

means for causing a gradual reduction in the effectiveness ofsaidheating means to cause a reduction in temperature from the top to thebottom of said article mold portion for solidification of the materialtherein from the top to the bottom.

5. A shell mold as in claim 4 including other heating means for thefilling mold portion to retain this portion above the meltingtemperature of the material to be cast during the solidification of thematerial in the article mold portion.

* 8 t i t

1. Apparatus for forming directionally solidified cast articles,including a mold having a filling cavity and an article forming cavityin horizontally spaced vertically positioned relation, and aninterconnecting passage between said cavities at the bottom, a chillplate closing the open upper end of the article forming cavity, heatingmeans for said filling cavity, and other heating means for said articleforming cavity, and means for selectively reducing the heating effect inportions of said other heating means between top and bottom thereof toproduce a controlled thermal gradient lower at the top than at thebottom of said article cavity.
 2. Apparatus as in claim 1, including acontainer to receive the mold with a granular material within thecontainer and supporting the mold.
 3. Apparatus as in claim 1 in whichthe article forming cavity of the mold has a single crystal selectoradjacent the upper end directly below the chill plate.
 4. A shell moldapparatus for casting of directionally solidified articles including anarticle mold portion having means adjacent the top for starting thecrystallization of the material therein as a single crystal, said moldportion having an open top end above said means, a chill plate extendingacross and closing said open top, a parallel filling mold portion openat the top for filling the mold, an interconnecting passage mold portionconnecting said article and filling portions at the bottoms thereof, thetop of said filling portion being higher than the open top end of thearticle portion, induction heating means around the article mold portionfor maintaining this portion at a temperature higher than the meltingtemperature of the material to be cast in the mold, and means forcausing a gradual reduction in the effectiveness of said heating meansto cause a reduction in temperature from the top to the bottom of saidarticle mold portion for solidification of the material therein from thetop to the bottom.
 5. A shell mold as in claim 4 including other heatingmeans for the filling mold portion to retain this portion above themelting temperature of the material to be cast during the solidificationof the material in the article mold portion.